In the past, an access control defined conforming to the IEEE (The Institute of Electrical and Electronic Engineers) 802.11 method, and the like, are widely known as medium access control of a wireless LAN system. Details of the IEEE 802.11 method is described in the International Standard ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, and the like. Networking in the IEEE 802.11 method is based on a concept of BSS (Basic Service Set). There are two kinds of BSS, a BSS defined by an infrastructure mode in which there exists a master control station such as an access point: AP, and an IBSS (Independent BSS) defined by an ad hoc mode which includes only a plurality of terminal stations: MT (Mobile Terminals).
An operation in the IEEE 802.11 at the time of the infrastructure mode is explained using FIG. 27. In the BSS of the infrastructure mode, there exists an access point performing coordination in a wireless communication system, and communication is performed between the access point and a terminal station MT existing around this access point. The access point transmits a control signal called a beacon at an appropriate time interval, and a terminal station MT capable of receiving this beacon recognizes that the access point exists in the neighborhood and further establishes a connection with this access point. FIGS. 27A to 27C describes a case where a communication station STA1 shown in FIG. 27A is an access point and a communication station STA0 shown in FIG. 27B is a terminal station MT. The communication station STA1 transmits a beacon at a fixed time interval as shown in FIGS. 27A to 27C.
The transmission time of the next beacon is informed in the beacon by a parameter called target beacon transmission time (TBTT), and the access point makes a beacon transmission procedure operated when the time becomes the TBTT. In addition, since the neighboring terminal station MT is able to recognize the next beacon transmission time by receiving the beacon and decoding an internal TBTT field thereof, there is also a case where power supply to a reception unit is turned off until a TBTT of the next time or of multiple times ahead and the station MT enters into a sleep state during a period of time considered to be not necessary to communicate with the access point (so-called intermittent reception operation). When information addressed to a specific communication station is contained in a beacon, a field conveying to the effect thereof to the concerned communication station is defined in the beacon, and the terminal station MT having received the beacon can know whether at present the access point retains information addressed thereto.
In FIGS. 27A to 27C, a case where the communication station STA0 receives a beacon of the communication station STA1 once every two times is shown as an example. FIG. 27C shows a state of a reception unit in the communication station STA0, in which a high level shows a state during reception operation and a low level shows a state during suspension of reception. The communication station STA0 operates the reception unit at a timing when the communication station STA1 transmits a beacon B1-0. However, when the reception of the beacon is completed, the communication station STA0 stops the operation of the reception unit, because there is no description in the beacon B1-0 that information addressed thereto is included. The communication station STA0 does not operate the reception unit when a next beacon B1-1 is transmitted, and operates the reception unit aiming at the time when the next beacon B1-2 after the beacon B1-1 is transmitted. In FIGS. 27A to 27C, a case where it is informed in this beacon B1-2 that information addressed to the communication station STA0 is included is referred to as an example.
The communication station STA0, which has recognized by receiving the beacon B1-2 that the information addressed thereto is included, transmits a PS-Poll packet in accordance with a predetermined transmission procedure in order to convey to the communication station STA1 that the information is recognized and the reception unit keeps operating. The communication station STA1 having received the packet transmits an information packet addressed to the communication station STA0 in accordance with the predetermined transmission procedure after recognizing that the communication station STA0 has started an operation of a receiving device. When receiving the information packet without error, the communication station STA0 transmits an ACK as a reception acknowledgement signal. Hereupon, information indicating that there is no more information included at present in the communication station STA1 is written in the information packet received by the communication station STA0, and the communication station STA0 having recognized the above again stops operation of the reception unit and makes a transition to the intermittent reception operation.
Further, when the access point transmits broadcast information, the access point performs countdown to decide when a broadcast message is transmitted in the future and informs the count value in a beacon. For example, when broadcast information is transmitted immediately after the beacon B1-2 in FIGS. 27A to 27C, a count value is written in the beacon B1-0, a count value is written in the beacon B1-1 and a count value 0 is written in a beacon B1-3, and without receiving a beacon signal each time, a terminal station MT can receive the broadcast information by referring to the relevant count value and operating the receiving device at a point of time when the count value becomes zero.
Next, an operation of the IEEE 802.11 at the time of an ad hoc mode is explained using FIGS. 28A to 28C. In the IBSS of the ad hoc mode, a terminal station (communication station) MT autonomously defines the IBSS after a negotiation is performed among a plurality of communication stations MT. When the IBSS is defined, a communication station group sets a TBTT at a fixed interval after a negotiation. When recognizing the TBTT by referring to an internal clock of its own, each communication station MT transmits a beacon after a delay of random time in case that it is recognized that nobody has transmitted a beacon yet. In FIGS. 28A to 28C, a case where two MTs that are the communication station STA0 and the communication station STA1 form an IBSS is shown as an example. FIG. 28A shows a packet that the communication station STA1 transmits and receives, FIG. 28B shows a packet that the communication station STA0 transmits and receives, and FIG. 28C shows an operation state of the reception unit of the communication station STA0 (in a state of high level reception operation and in a state of low level reception suspension). With respect to a beacon in this case, a communication station MT of either the communication station STA0 or the communication station STA1, which belongs to the IBSS, transmits a beacon each time the TBTT has come.
In the IBSS, also there is a case in which a communication station MT turns off a power supply to a transmission-reception unit and enters a sleep state depending on necessity. When a sleep mode is applied in the IBSS, a period of time for some time from the TBTT is defined as an ATIM (Announcement Traffic Indication Message) window in the IEEE 802.11. All the communication stations MT belonging to the IBSS operate reception units during the period of time of the ATIM window, and also a communication station MT basically operating in the sleep mode is capable of receiving during this period of time.
When each communication station MT retains information addressed to another station, the fact that the station retains the information addressed to the above another station is notified to a receiving side by transmitting an ATIM packet to the above-described another station during the period of time of the ATIM window after a beacon is transmitted. The communication station MT having received the ATIM packet or the communication station MT having transmitted the beacon keeps a reception unit operating until the next TBTT.
In FIGS. 28A to 28C, when it becomes the first TBTT, each communication station MT of the STA0 and STA1 operates a back-off timer while monitoring a state of the medium over a random time. A case in which a timer of the communication station STA0 finishes counting at the earliest and the communication station STA0 transmits a beacon is shown in an example of FIGS. 28A to 28C. Since the communication station STA0 has transmitted a beacon, the communication station STA1 having received this beacon does not transmit a beacon. Further, the communication station STA0 keeps the reception unit operating until the next beacon is transmitted, because the STA0 has transmitted the beacon.
At the next TBTT, the communication station STA1 transmits a beacon in accordance with a procedure of random back-off. At this time, although the communication station STA0 operates the reception unit during the period of time defined by the ATIM window, the station STA0 receives no information from another station during this period and thereby the communication station STA0 stops the reception unit immediately after the period of the ATIM window is over and makes a transition to a sleep state until the next TBTT. Even at the next TBTT, the communication station STA1 transmits a beacon again in accordance with the procedure of random back-off. At this time, since the communication station STA0 receives an ATIM message from the communication station STA1 while keeping the receiving device operating during the period of time defined by the ATIM window, the communication station STA0 keeps the reception unit operating after the period of the ATIM window is over and receives information transmitted from the communication station STA1. Since an ACK that is a reception acknowledgement is received in response to the ATIM message, the communication station STA1 tries to transmit a data packet by activating the procedure of random back-off starting from a point of time that the ATIM window is ended after confirming that the communication station STA0 recognizes the reception. After that, both the communication stations STA1 and STA0 keep the reception units operating until the next beacon transmission.
As described above, in a wireless communication system (wireless LAN and the like) of related art, a communication station which has no information to receive turns off a power supply to a transmission-reception unit until the next TBTT and can reduce power consumption.
An example of wireless communication processing in related art using such beacon is disclosed in Published Japanese Patent Application No. H8-98255.
When the above communication control is performed, the following problems exist.
Accumulation of Data Occurring on Transmission Side
An appearance in which a packet transmission delay occurs in a system of related art is shown in FIGS. 29A to 29E.
FIG. 29A is a packet sent from an upper layer of the communication station STA1, FIG. 29B is a packet (including a beacon) transmitted and received by a MAC layer of the communication station STA1, FIG. 29C is a packet transmitted and received by a MAC layer of the communication station STA1, and FIG. 29D is a packet received by the communication station STA1 and sent to an upper layer. In addition, FIG. 29E shows a reception operation state in the reception unit of the communication station STA1. Once a reception unit is stopped, a communication station does not operate the reception unit until the next beacon transmission time. Therefore, when data is transferred from the communication station STA1 to the communication station STA0 and in such a case that information is periodically passed down from an upper layer of the communication station STA1, the next time when the communication station STA0 having made the reception unit once stopped operates the reception unit is at the time when the communication station STA1 transmits a beacon, and data from D1 to D5 passed from an upper layer of the communication station STA1 during this period are accumulated within the communication station STA1 as shown in FIGS. 29A to 29E, for example. The accumulation of data within the transmission and reception unit causes an increase (increase of latency) in round trip time (RTT) between the reception and transmission units at an application level, and there occurs a problem that throughput reaches a ceiling when ARQ using a sliding window is performed. Moreover, in order to avoid an overflow, a buffer as big as one capable of sustaining the data accumulation within the communication station STA1 that is the transmission station is needed, which causes a problem of limitation regarding hardware.
Increase in Idle Reception Period Occurring on Reception Side
Next, an idle reception period that occurs on a reception side is explained using FIGS. 30A to 30F. FIG. 30A through 30E are the same as FIGS. 29A through 29E, and FIG. 30F shows an idle period.
In addition, although a method of keeping a reception unit operating even after the reception of information is employed in related art in order to solve the above-described problem, the reception unit becomes always operating in this case as shown in FIGS. 30A to 30F, for example. Although the accumulation of data in the transmission station STA1 is certainly resolved, the reception unit is uselessly operated in the communication station STA0 that is the reception side and a problem remains from a viewpoint of power consumption since there exist a number of idle periods (period shown with an arrow in FIG. 30F) in which data reception is not performed actually though the reception unit keeps operating.
Latency of Broadcast Information
Further, although broadcast information transmitted and received in an infrastructure mode can be conveyed even if a communication station MT receives beacon information only once in several times, it is necessary to keep the transmission of the broadcast information waiting from the time when data is passed from an upper layer until countdown is ended when an access point AP intends to broadcast the data passed from the upper layer, and there occurs such a problem that a delay until the broadcast information is actually transmitted becomes large.
The present invention is made in view of those problems and aims at solving problems such as accumulation on a transmission side and delay on a reception side at the time of transmission in a communication system such as a wireless LAN system.